The 65-year-old Secretary of State's spokesman said the clot was found during a follow-up exam related to the concussion she sustained earlier this month when she fainted due to dehydration; Clinton was suffering from a stomach virus and has been sidelined from work for the last three weeks.

Clinton is expected to remain at New York Presbyterian Hospital for the next two days so physicians can treat her with anti-coagulants and keep an eye on her.

Philippe Reines, deputy assistant secretary of state, said in a statement, "In the course of a follow-up exam today, Secretary Clinton's doctors discovered a blood clot had formed, stemming from the concussion she sustained several weeks ago. She is being treated with anti-coagulants and is at New York Presbyterian Hospital so that they can monitor the medication over the next 48 hours. Her doctors will continue to assess her condition, including other issues associated with her concussion. They will determine if any further action is required."